This invention relates to a method for operating a laser-based detection system and more particularly to a method to determine the alignment of the transmitter and receiver fields of view of a light detection and ranging (LIDAR) system. LIDAR systems are used to measure distances and properties of distant objects. Among the many uses of LIDAR are the measurement of clouds, aerosols, water vapor, pollutants, and forest growth and health. In a LIDAR system, the alignment of a transmitted laser beam with respect to the field of view of the receiver optics is critical, particularly if the laser beam divergence is a large fraction of the total field of view of the receiver optical system. If the field of view of the receiver optical system is small, the alignment is even more critical. Although the initial alignment of all of the transmitter and receiver optics can be performed in a laboratory using precision optical alignment tooling, (interferometers, autocollimators, and other apparatus well understood by those skilled in the art), these tools are usually not available in the field during actual use of the LIDAR system. In order to allow verification of proper alignment in the field and in-field realignment when necessary, we have invented a laser beam position detector as a integral part of the receiver optics of a LIDAR system that, in combination with a special lateral-transfer retro-reflector/attenuator assembly, enables the rapid, in-field checking of the co-alignment of the transmitted laser beam with respect to the receiver telescope field of view (FOV). It also enables in-field adjustment of the relative alignment of the transmitter and receiver systems to achieve the necessary degree of alignment for LIDAR measurements.
Paine et al. (U.S. Pat. No. 3,574,467) reports a method and apparatus particularly suited for use in aligning an optic system, such as an astronomical telescope, employed as a laser beam projector for projecting a beam of laser light against a celestial target and including therein an arrangement of optically related lenses and mirrors by which light emanating from a celestial target is brought in focus to form an image of the target within the focal plane of the optic system, and characterized by the utilization of a beam splitter having a reflecting surface including a microscopic opening disposed within the path of a projected laser beam, as well as within the path of the light being brought in focus, whereby the laser beam is projected through the optic system toward the target, while the light emanating from the target is brought in focus in the focal plane of the system and then redirected by the reflecting surface of the beam splitter to a second optic system for imaging both the target and the opening formed in the beam splitter for thereby accommodating a visual detection of optical alignment of the system for assuring alignment of the system relative to the target.
Kadrmas (U.S. Pat. No. 3,781,552) reports a transmitting and receiving telescope for use in generalized electromagnetic radiation communication systems, including atmospheric probing systems. The telescope optics, electromagnetic radiation source laser and receiver are coaxially aligned along the telescope axis. The telescope can be constructed with one received field of view or with a plurality of received fields of view. The telescope mirrors have apertures along the telescope axis to allow alignment laser pulse or CW radiation to travel along the telescope axis without being reflected. The preferred electromagnetic radiation source is a laser which can operate in both TEM00 and TEM01 modes. The TEM00 mode is employed for alignment purposes since the energy of this mode is concentrated along the axis. The TEM01 mode is used for data acquisition because the energy of this mode is concentrated in a donut shaped region having its hole centered on the axis. Constant intensity illumination is produced in the viewed area during data acquisition by separating the donut into inner and outer annuli along the line of maximum intensity and imaging the two beams to provide 100 percent overlay at the range of interest. A preferred use for this telescope is in atmospheric probing LIDAR systems for measurements of the motion and concentration of the atmospheric environment, particularly pollution measurements. This LIDAR system uses a real time data processing system employing 800-megabit analog to digital converters and a correlation system to transform the acquired data into useable form in real time.
Feichtner (U.S. Pat. No. 5,825,464) reports a calibration system and method including isotropically diffusing high-intensity light emitted from a detection apparatus, with the diffused light being coupled to at least one optical line, typically an optical fiber. In the preferred embodiment, the apparatus to be calibrated is a light detection and ranging (LIDAR) apparatus and the isotropic diffusion is achieved by means of an integrating sphere. For calibrating distance calculations, the optical fiber includes a reflecting mechanism, such as Bragg gratings, at a fixed and known distance from the input end of the optical fiber. The reflecting mechanisms simulate the presence of a remote object at the known distance. For a velocity calculation, a frequency offset may be introduced to the light propagating through the optical fiber, thereby simulating a Doppler shift induced by a moving object. The simulations of the presence and movement of remote objects are used to calibrate the apparatus.
Ray and Sedlacek (U.S. Pat. No. 6,608,677) reports a method and apparatus for remote, stand-off, and high efficiency spectroscopic detection of biological and chemical substances. The apparatus includes an optical beam transmitter which transmits a beam having an axis of transmission to a target, the beam comprising at least a laser emission. An optical detector having an optical detection path to the target is provided for gathering optical information. The optical detection path has an axis of optical detection. A beam alignment device fixes the transmitter proximal to the detector and directs the beam to the target along the optical detection path such that the axis of transmission is within the optical detection path. Optical information gathered by the optical detector is analyzed by an analyzer which is operatively connected to the detector.
In order to allow verification of proper alignment in the field and in-field realignment when necessary, this invention employs a laser beam position detector as a integral part of the receiver optics of a LIDAR system that, in combination with a special lateral-transfer retro-reflector/attenuator assembly, enables the rapid, in-field checking of the co-alignment of the transmitted laser beam with respect to the receiver telescope field of view (FOV). This invention also enables in-field adjustment of the relative alignment of the transmitter and receiver systems to achieve the necessary degree of alignment for LIDAR measurements.